This invention relates to a probe for sensing pH of the blood although the probe constructed in accordance with the principles of the present invention could be used for sensing other blood gases or electrolytes.
The concept of mounting a blood gas sensitive dye on the end of an optical fiber, exciting the dye with light passing through the optical fiber and measuring the partial pressure of the blood gas by measuring some aspect of the excited fluorescent dye is known. See, for example, U.S. Pat. No. 31,879, issued May 7, 1985, Lubbers et al, and U.S. Pat. No. 4,200,110, issued Apr. 29, 1980, Peterson et al. This concept is of great importance to the medical profession, for its permits the real time monitoring of a patient's condition during medical/surgical procedures.
In spite of the very considerable need for a probe tiny enough to be insertable into the blood vessels of a patient, no probe has enjoyed any measurable commercial success.
An objective of the present invention has been to provide an improved probe and a method of making it.
Another more specific objective of the present invention has been to provide a method, and an article produced from the method, for placing a sensor/gel on the end of a tiny optical fiber. The optical fiber in the illustrated form of the invention is 0.009" in diameter.
The objectives of the invention are obtained by providing a sleeve around the optical fiber. Initially, the ends of the sleeve and optical fiber coincide, that is to say, they lie in the same plane. The fiber/sleeve combination is placed into a drop of sensor matrix, i.e., a mixture containing a fluorescent indicator, the monomer, and polymerization initiator, which is curable to provide a sensor gel. The fiber is pulled back with respect to the sleeve thereby creating a pocket and a vacuum in the pocket which is immediately filled by the flow of the sensor matrix into the thus formed pocket. Thus, it is that a very tiny volume (a cylinder approximately 0.009" long and 0.010" diameter) of the sensor matrix is mounted on the end of an optical fiber without the possibility of oxygen occurring at the interface between the end of the fiber and the sensor matrix. The sensor matrix is then cured to provide a stable sensor gel on the end of the probe.
Curing the sensor matrix using conventional heating means to provide the desired sensor gel requires heating the sensor matrix in an oven for three to five hours. In accordance with one aspect of the present invention, the sensor matrix is cured by subjecting it to ultraviolet light for a period of about twenty seconds. The particular combination of components in the sensor which permits this twenty-second ultraviolet light cure includes a derivative of HOPSA (HOPSA is an acronym for 8-Hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt), acrylamide monomers, and an azo-polymerization initiator.
It was necessary to prepare a derivative of the HOPSA indicator that contained an alkene function which would copolymerize with the acrylamide monomers of which the hydrogel portion of the sensor gel is made. The hydrogel is the support material in which the indicator is copolymerized and held while still allowing molecular contact with the analyte to be measured. Through numerous tests, a HOPSA derivative was found which appeared to be held very strongly within the polyacrylamide hydrogel upon curing of the sensor matrix. That derivative, di-substituted HOPSA (2-propenyl)sulfonamido, which for ease of reference will be referred to hereinafter as "di-substituted HOPSA," has the following structural formula: ##STR1##
Another aspect of the present invention involves creating and maintaining an oxygen-free atmosphere throughout the curing (polymerization) process in order to eliminate the presence of oxygen which competes against the acrylamide monomers for the initiator. This not only slows the cure of the polyacrylamide, it may also detrimentally affect the sensitivity of the sensor gel.
Another feature of the present invention resides in the mounting of a fiber in a Y-connector which, in turn, can be connected to a catheter through which the optical fiber and sleeve, with the sensor on its end, can be inserted into the blood vessel of a patient.